A process of the initially mentioned type is suitable especially for the production of reflectors. Such a process is described, for example, in DE 40 08 405 C1, U.S. Pat. No. 5,154,943 and U.S. Pat. No. 5,236,511, the disclosures of which are hereby incorporated by reference.
Reflectors are generally composed of arched, approximately dome-shaped substrates ("domes"), in most cases made of glass, with an inside reflective coating, for example, a cold-light mirror coating (e.g., as employed for dental mirrors). In particular, glass blanks shaped by pressing are usually used as substrates. The outer surface of the shaped glass blanks are provided with lamp shafts, so-called dome necks, for the electrical connections. The reflective coating can comprise a metallic layer or, if a special spectral gradient of reflectance is desired, the reflective coating can comprise a dielectric layer. The optical quality requirements of such layers, especially also of the uniformity of the coating, are high.
From DE 40 08 405 C1 and its corresponding U.S. Pat. Nos. 5,154,943 and 5,236,511, it is known to produce such reflectors by a plasma-CVD process (PCVD). Plasma-CVD processes and also the production of dielectric layer systems with a given spectral gradient, are known in the art and described diversely in the patent literature and other literature. However, until the invention of DE 40 08 405 C1, it was not possible to use plasma-CVD processes for production of layer systems of high optical quality on greatly arched substrates, such as, e.g., production of reflective coatings on domes to be used in the production of reflectors. For the production of a uniform coating on a greatly arched substrate, it is necessary, if an expensive relative motion between substrate and coating zone is to be avoided, that the layer-forming reaction zone extend over the entire surface to be coated during the coating process. For this purpose, high-volume plasma zones are necessary, since for complete covering of the entire surface to be coated, the region enclosed by the greatly arched substrates also has to lie inside the plasma area.
However, as the thickness of the plasma zone over a surface to be coated increases, the probability also increases that, by a so-called homogeneous reaction, particles will form in the gas-filled space which will then deposit as "glass soot." This particle formation leads to layer cloudiness, making the layers unusable for optical applications. Particle formation occurs especially in the edge areas of the plasma zone, inside which the power density of the plasma drops below a critical value.
To suppress the above-described particle formation in the gas-filled space during plasma-CVD coating of greatly arched substrates, it is proposed in DE 40 08 405 C1, to limit the thickness of the gas layer to be reacted over the surface to be coated by the use of a so-called displacement element. The displacement element penetrates into the inner space enclosed by the dome, for example, when the dome is to be coated on the inside, and exhibits a shape corresponding substantially to the shape of an arched substrate. As a result, the extent of the glass soot formation occurring in the gas layer during the plasma phase remains harmless for the desired optical layer quality. The displacement element of DE 40 08 405 C1 thus has the object, among other things, of masking the low power density edge areas of the plasma zone inside which glass soot formation especially occurs. Due to the displacement element, the layer-former molecules are trapped and held in a solid layer on the surface. Without the opposing surface of the displacement element, the layer-forming molecules would penetrate into the edge areas of the plasma zone and contribute to particle production. The supply of reaction gases takes place according to D 40 08 405 C1 through the displacement element which, for this purpose, exhibits a duct that ends in a central gas outlet at the front surface thereof. The gas inlet is thus positioned so as to face the vertex of the dome at a distance from the surface to be coated.
In CVD processes, reactions gases usually are conveyed so that they flow slowly and continuously into the reaction chamber containing the substrate(s) to be coated. An initial consideration was the fact that turbulences in the gas streams resulted in layer unevenness. Turbulences should therefore be avoided at all costs. Thus, the displacement element described in DE 40 08 405 C1 is used to not only delimit the reaction zone, but to also produce a slow and continuous flow of reaction gases along the surface to be coated.
A disadvantage associated with the known process is that it is expensive to perform. The displacement element, which as a rule is positioned at only a small distance from the surface to be coated, is coated along with the substrate to practically the same thickness. Therefore, to prevent a gradual closing of the gap between the surface of the displacement element on the substrate side and the inner surface of the substrate, it is necessary to regularly remove the deposited layer. Especially when using a plasma-pulse-CVD process, gradual reduction of the distance between the displacement element and the substrate, i.e., reduction of the thickness of the gas layer to be reacted, results in an undesirable reduction of the rate of coating during the coating process. Another drawback of the known process is that the displacement element is very expensive to produce, since it has to be matched, in each case, to the shape of the substrate to be coated.